Problem with trunk sip or iax betweeen countryes

I had tested trunks sip and iax between 2 asterisk in many places its work very good, but now a had used the similar trunks between 2 box to Panama on Colombia, the problem is when there are 2 or more calls, the voice is very bad and it cuts.

I think that problem is the traffic on teh international net, but with one call it works fine, and 2 calls are very little to fill a channel, any idea??

Thanks for your answers, dicko you have rigth, but i am searching a solution, chiling my bandwidth in Colombia is 2 M and Panama 1 M, but the problem is today too, Sunday without any traffic in the company, codecs all posibles ullaw, allaw, gsm, g711a, etc. Thanks.

Do you have a symmetric DSL service there? The most common here in Spain is having a asymmetric DSL (ADSL), with lower upload than download. For example, I have 20Mb/1Mb ADSL.
I tell you this because the upload speed is as important as the download speed to have a good voice connection.